Variable pitch propeller systems incorporate mechanisms to protect the systems against uncommanded blade angle excursions towards low pitch or low blade angle, typically in the event of hydraulic failure. Propeller designs which use pitch lock systems also allow for the use of a simple pitch change actuator. Accordingly, hydraulic pressure required to adjust propeller blade pitch or angle may be supplied directly to the coarse pitch and fine pitch chambers of the pitch change actuator usually found in such systems, on command from a propeller control. Many propeller systems, are controlled via a hydromechanical system. These systems typically include a flyweight governor, a differential gear set for blade angle feedback, and numerous hydraulic valves, cams, and levers.
An electronic control system is typically operable to control and monitor blade angle thereby changing blade angle as demanded by flight conditions. In the typical case, such commands are implemented by an electronically operated valve which directs and cuts off hydraulic fluid to the coarse and fine pitch chambers of the pitch change actuator.
In the event that there is an electronic failure within the propeller control system or loss of aircraft electrical power, a backup system must be deployed, which system is typically mechanical, and will adjust or maintain the blade angle in accordance with flight conditions. In this manner, electronic malfunctions, which could cause blade angle to send the propeller into overspeed conditions and high negative thrust can be avoided.
In currently available mechanical propeller control systems, with mechanical backups, the overall system size and complexity can be problematic with regard to important weight and cost, and reliability considerations.
There exists a need, therefore, for an electronic propeller control system with a mechanical backup, which is reliable and provides due consideration to weight and complexity for increasing flight efficiency and decreased costs, respectively.